FIGS. 1A and 1B depict conventional magnetic elements 10 and 10′. Such conventional magnetic elements 10/10′ can be used in non-volatile memories, such as MRAM. The conventional magnetic element 10 is a spin valve and includes a conventional antiferromagnetic (AFM) layer 12, a conventional pinned layer 14, a conventional nonmagnetic spacer layer 16 and a conventional free layer 18. Other layers (not shown), such as seed or capping layer may also be used. The conventional pinned layer 14 and the conventional free layer 18 are ferromagnetic. Thus, the conventional free layer 18 is depicted as having a changeable magnetization 19. The conventional nonmagnetic spacer layer 16 is conductive. The AFM layer 12 is used to fix, or pin, the magnetization of the pinned layer 14 in a particular direction. The magnetization of the free layer 18 is free to rotate, typically in response to an external magnetic field. The conventional magnetic element 10′ depicted in FIG. 1B is a spin tunneling junction. Portions of the conventional spin tunneling junction 10′ are analogous to the conventional spin valve 10. However, the conventional barrier layer 16′ is an insulator that is thin enough for electrons to tunnel through in a conventional spin tunneling junction 10′. Note that only a single spin valve 10 is depicted, one of ordinary skill in the art will readily recognize that dual spin valves including two pinned layers and two nonmagnetic layers separating the pinned layers from the free layer can be used. Similarly, although only a single spin tunneling junction 10′ is depicted, one of ordinary skill in the art will readily recognize that dual spin tunneling including two pinned layers and two barrier layers separating the pinned layers from the free layer, can be used. More recently, structures having two pinned layers and two layers, one barrier and one conductive, separating the pinned layers from the free layers have been developed, particularly for use when exploiting spin transfer in programming.
Depending upon the orientations of the magnetization 19/19′ of the conventional free layer 18/18′ and the conventional pinned layer 14/14′, respectively, the resistance of the conventional magnetic element 10/10′, respectively, changes. When the magnetization 19/19′ of the conventional free layer 18/18′ is parallel to the magnetization of the conventional pinned layer 14/14′, the resistance of the conventional magnetic element 10/10′ is low. When the magnetization 19/19′ of the conventional free layer 18/18′ is antiparallel to the magnetization of the conventional pinned layer 14/14′, the resistance of the conventional magnetic element 10/10′ is high.
To sense the resistance of the conventional magnetic element 10/10′, current is driven through the conventional magnetic element 10/10′. Typically in memory applications, current is driven in a CPP (current perpendicular to the plane) configuration, perpendicular to the layers of conventional magnetic element 10/10′ (up or down, in the z-direction as seen in FIG. 1A or 1B). Based upon the change in resistance, typically measured using the magnitude of the voltage drop across the conventional magnetic element 10/10′, the resistance state and, therefore, the data stored in the conventional magnetic element 10/10′ can be determined.
In conventional MRAM, the conventional magnetic element 10/10′ is written using an in-plane magnetic field that is approximately forty-five degrees from the axis in which the magnetization 19/19′ lie. This magnetic field is typically provided by driving current through two write lines (not shown) which are oriented perpendicular and which cross in the region of the conventional magnetic element 10/10′. Depending upon the direction of the magnetic field, the magnetization 19/19′ of the free layer 18/18′ can be switched to have an equilibrium position parallel or antiparallel to the magnetization of the pinned layer 14/14′.
Although the conventional magnetic elements 10/10′ can be used to store data in an MRAM, one of ordinary skill in the art will readily recognize that there are a number of drawbacks. Of these, the primary issues include poor write selectivity and a high write current required to write to the conventional magnetic elements 10/10′. Typically, a magnetic cell includes the conventional magnetic element 10/10′ and other element(s), such as a selection transistor. Poor write selectivity results in memory cells in addition to the desired memory cell being written. These unintentionally written cells are typically adjacent to the memory cell desired to be written. During manufacturing, defects may be introduced into elements within the memory cells. Manufacturing also results in variations in the size and shape of the conventional magnetic elements 10/10′, as well as other portions of the memory cell. The defects and variations in the memory cell size and shape cause variations in the internal demagnetizing field. The magnetic field produced within the memory cell (internal magnetic field), which includes the demagnetizing field and the applied field at the memory cell, may vary widely from cell to cell. The variations in the internal magnetic field mean that the magnetic field required to write a particular magnetic cell (required write field) varies from cell to cell. Variations in the required write field mean that neighboring cells may be affected by the magnetic field applied to write to a particular cell. Consequently, unintentional cell writing of neighboring cells may occur. Thus, defects and variations in the memory cell size and shape may result in a large distribution in the required write field and unintentional cell writing for neighboring cells.
Writing to conventional magnetic cells may also require a larger write current, which is undesirable. As discussed above, there is large distribution in the required write field. Consequently, a higher applied magnetic field is often generated to ensure that the desired memory cells are written. A higher applied magnetic field requires a higher current to be driven through the write lines. This higher current is undesirable, for example due to increased power consumption. In addition, the possibility of unintentional cell writing may be increased by the higher magnetic field.
Accordingly, what is needed is a system and method for providing a magnetic memory element that can be switched using a lower current and that has improved switching characteristics. The present invention addresses such a need.